This class is used to provide a free implementation of the EvalFunc exec function given
implementation of the Accumulator interface. Instead of having to provide a redundant
implementation, this provides the base exec function for free, given that the methods
associated with the Accumulator interface are implemented. For information on how to
implement Accumulator, see Accumulator.

exec

This callback method must be implemented by all subclasses. This
is the method that will be invoked on every Tuple of a given dataset.
Since the dataset may be divided up in a variety of ways the programmer
should not make assumptions about state that is maintained between
invocations of this method.